Tire Size Box

345/45R20 vs 345/55R20

345/55R20 compared with 345/45R20

+8.4% diameter, 0.0" width

345/55R20 is 34.94" tall vs 32.22". More than the 3% rule of thumb, so check carefully before switching.

32.2"20" rim6.1"13.6" wide
345/45R20 (solid) compared with 345/55R20 (dashed outline), drawn to scale

With 345/55R20 tires, when your speedometer says 60 mph you are really going

65.1 mph

345/55R20 is +2.72 inches (+8.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

345/45R20345/55R20Difference%
Overall diameter32.22"34.94"+2.72"+8.4%
Section width13.58"13.58"0.00"0.0%
Sidewall height6.11"7.47"+1.36"+22.2%
Circumference101.24"109.77"+8.53"+8.4%
Revolutions per mile625.9577.2−48.7−7.8%

Speedometer difference

If your vehicle came with 345/45R20 and you fit 345/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.4 mph+3.4 mph
50 mph54.2 mph+4.2 mph
60 mph65.1 mph+5.1 mph
65 mph70.5 mph+5.5 mph
70 mph75.9 mph+5.9 mph
75 mph81.3 mph+6.3 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 345/45R20 or 345/55R20?
345/55R20 is taller by 2.72 inches (34.94" vs 32.22"). 345/45R20 is the same width.
Can I replace 345/45R20 with 345/55R20?
It is not recommended without checking carefully. The diameter difference is +8.4%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 345/55R20 change my speedometer?
When your speedometer reads 60 mph with 345/55R20 fitted (calibrated for 345/45R20), your real speed is 65.1 mph. Your odometer will under-count miles by about 8.4%.
Will 345/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.36 inches of ground clearance at the axle.

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